The present invention relates to a method and apparatus for shielding electromagnetic integrated circuits from external magnetic fields.
A conventional integrated circuit (IC) package typically comprises (1) an IC chip or die including a plurality of input/output terminals; (2) a support for the chip, such as a pad, substrate or leadframe, including electrically conductive leads; (3) electrical connections such as wire bonds or conductive bumps for electrically connecting the input/output terminals of the chip with the electrically conductive leads; and (4) a material for encasing or encapsulating the chip, the support and the electrical connections while leaving portions of the leads accessible outside the casing or encapsulation. Fabrication of such a conventional IC package requires attaching the IC chip to the support, connecting the input/output terminals of the chip to the electrically conductive leads, and encapsulating the IC chip, the support and the electrical connections in, for example, a plastic package.
Recently, very high-density magnetic memories, such as magnetic random access memories (MRAMs), have been proposed to be integrated together with CMOS circuits. Magnetic random access memories employ one or more ferromagnetic films as storage elements. A typical multilayer-film MRAM includes a plurality of bit or digit lines intersected by a plurality of word lines. At each intersection, a ferromagnetic film is interposed between the corresponding bit line and word line to form a memory cell.
When in use, an MRAM cell stores information as digital bits, the logic value of which depends on the states of magnetization of the thin magnetic multilayer films forming each memory cell. As such, the MRAM cell has two stable magnetic configurations, high resistance representing, for example, a logic state 0 and low resistance representing, for example, a logic state 1. The magnetization configurations of the MRAMs depend in turn on the magnetization vectors which are oriented as a result of electromagnetic fields applied to the memory cells. The electromagnetic fields used to read and write data are generated by associated CMOS circuitry. However, stray magnetic fields, which are generated external to the MRAM, may cause errors in memory cell operation when they have sufficient magnitude.
Very high-density MRAMs are particularly sensitive to stray magnetic fields mainly because the minuscule MRAM cells require relatively low magnetic fields for read/write operations which, in turn, depend upon the switching or sensing of the magnetic vectors. These magnetic vectors are, in turn, easily affected and have the magnetic orientation changed by such external stray magnetic fields.
To diminish the negative effects of the stray magnetic fields and to avoid sensitivity of MRAM devices to stray magnetic fields, the semiconductor industry could introduce memory cells requiring higher switching electromagnetic fields than a stray field which the memory cells would typically encounter. However, the current requirements for operating such memory cells is greatly increased because higher internal fields necessitate more current. Thus, both the reliability and scalability of such high current devices decrease accordingly, and the use of MRAMs which may be affected by stray magnetic fields becomes undesirable.
Accordingly, there is a need for an improved magnetic memory structure and a method of forming it, which shields against external magnetic fields. There is also a need of a packaging device for encasing a magnetic random access memory IC chip which reduces the effects of external magnetic fields on internal memory cell structures and operations. There is further a need for minimizing the cost of a packaging which shields a magnetic random access memory IC chip from external magnetic fields.
The present invention provides a method and apparatus which provide a packaging device for magnetic memory structures, such as MRAMs, which shields such memory structures from external magnetic fields. The invention employs a magnetic shield, preferably formed of non-conductive magnetic oxides, which either partially contacts or completely surrounds an integrated circuit chip which includes such magnetic memory structures.
These and other features and advantages of the invention will be more clearly apparent from the following detailed description which is provided in connection with accompanying drawings and which illustrates exemplary embodiments of the invention.